Environmental controls on the formation and isotopic composition of a laminated tufa in Red Butte Canyon, Utah

Stable isotope variations in laminated tufa deposits have provided valuable archives of paleoenvironmental conditions; however few detailed investigations on how specific environmental variables are integrated into a tufa's isotopic signal have been completed. To better understand environmental controls on tufa isotopic composition in Red Butte Canyon, Utah, one year of geochemical monitoring of the tufa-bearing stream Parley's Fork, groundwater and carbonate collected on artificial substrate was performed. Seasonal variations in temperature, Mg/Ca and Sr/Ca in the stream were not captured in the Mg/Ca or Sr/Ca of the temporally corresponding carbonate. Degassing of CO2 from the stream, possibly by ebullition of gas bubbles, is the most important process leading to tufa deposition, compared to temperature changes and biological activity. Equilibrium fractionation from stream DIC accounts for the delta 13 C of tile calcites during the summer and early fall, but in the winter and spring tile calcites are more enriched, possibly due to the microenvironmental influence of photosynthetic organisms. Seasonal variations of delta 18 O in Parley's Fork are correlated with stream discharge, and temperature dependent equilibrium fractionation appears to account for the observed variations in tile calcite delta 18 O. Based on these observations, periods of enriched delta 13 C and delta 18 O in tufa laminae likely correspond to cool or wet climatic intervals. Stable isotope and trace element analysis and radiocarbon dating was completed on two tufa specimens collected from Parley's Fork. The largest tufa specimen spanned the time interval from A.D. 990 to 1990. Evidence of the past 150 years of fossil fuel emission induced changes to the delta 13 C of the atmosphere exists in the tufa delta 13 C signature. The Little Ice Age (1600 to 1800) portion of the tufa displays large oscillations in isotopic composition, possibly indicating numerous rapid (decade-scale) shifts in local climate. In the early part of the last millennium, suppressed isotopic variations appear to correspond with significant drought events. The Medieval Warm Period (800 to 1200) is evidenced by a distinct change to lighter values of delta 13 C and delta 18 O, and a corresponding decrease in the Mg/Ca and Sr/Ca ratios.